The city of Chicago bans private citizens from keeping handguns in their homes. So, too, did the District of Columbia, before the Supreme Court's 2008 ruling, in District of Columbia v. Heller, that the District law violated the Second Amendment guarantee of the right to keep and bear arms. Should the Chicago law be treated any differently, as a constitutional matter, than the District ban, because of the District's unique status as a federal enclave? Or, to put the question in legal terms: Is the Second Amendment among the protections in the Bill of Rights that also constrain state and local governments? Acting on the final day of the 2009 term and splitting 5 to 4, the Supreme Court said Monday that it is.

The court's decision in McDonald v. City of Chicago is both correct and potentially troubling. It is correct because the general tendency of the court has been to find that the amendments in the Bill of Rights -- guarantees of free speech and freedom of religion, protections against unreasonable search and seizure, prohibitions on cruel and unusual punishment -- apply to state and local governments through the 14th Amendment's due process clause. Some scattered and relatively minor provisions in the Bill of Rights -- such as requiring a jury trial in civil cases or prohibiting excessive fines -- are not deemed to have been "incorporated" by the 14th Amendment, but most are. Having decided that the Second Amendment protects an individual right to bear arms, it would have been odd for the court to determine that state governments can infringe on that right.

The ruling is potentially troubling, however, for the same reason that Heller was: the threat that courts will step in to overturn reasonable legislative judgments that public safety requires certain restrictions on gun ownership. Accepting the court's holding that the Second Amendment protects a right to gun ownership in self-defense does not mean that judges should willy-nilly strike down every infringement on that right. The majority, in an opinion by Justice Samuel A. Alito Jr., emphasized that the court in Heller recognized that the right to keep and bear arms is not "a right to keep and carry any weapon whatsoever in any manner whatsoever and for whatever purpose": The court approvingly cited laws prohibiting carrying firearms in schools or government buildings, bans on gun possession by felons and regulations on gun sales.

"We repeat those assurances here," Justice Alito wrote. "Despite [Chicago's] doomsday proclamations, incorporation does not imperil every law regulating firearms." We hope not. Monday's ruling opens the door wide to an inevitable series of legal challenges that will tempt judges to substitute their judgments about gun control for that of legislators. They should act with proper restraint and respect for the limits of the judicial role.